U.S. Secretary of State John Kerry, Britain's Foreign Secretary Philip Hammond, Russian Foreign Minister Sergei Lavrov and former EU foreign policy chief Catherine Ashton, from left, are on their way to a ... more

U.S. Secretary of State John Kerry, Britain's Foreign Secretary Philip Hammond, Russian Foreign Minister Sergei Lavrov and former EU foreign policy chief Catherine Ashton, from left, are on their way to a meeting during their nuvlear talks on Iran, in Vienna, Austria, Monday, Nov. 24, 2014. (AP Photo/Joe Klamar, Pool) less

Photo: Joe Klamar, POOL

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German Foreign Minister Frank-Walter Steinmeier, French Foreign Minister Laurent Fabius and Chinese Foreign Minister Wang Yi, from left, sit at the negotiation table during their talks on Iran, in Vienna, ... more

VIENNA - A yearlong effort to seal a nuclear deal with Iran fizzled Monday, leaving the U.S. and its allies little choice but to declare a seven-month extension in hopes that a new deadline will be enough to achieve what a decade of negotiations have failed to do - limit Tehran's ability to make a nuclear weapon.

Pushback from critics in Congress followed almost immediately, with powerful Republicans saying that Iran is trying to buy time.

U.S. Secretary of State John Kerry and other Western foreign ministers defended the add-on time as the best way forward.

"We would be fools to walk away," Kerry declared.

But a week of tough maneuvering appeared to have achieved little more than agreement to keep on talking. Negotiators will now strive to nail down by March 1 what Iran and the six world powers it is negotiating with must do, and by when. A final agreement is meant to follow four months later.

Members of the new Republican-controlled Congress to be sworn in early next year threatened to impose additional sanctions on Iran and may well have enough votes to overturn an expected veto by President Barack Obama.

"The one thing the Iranians didn't have was time, and now they have 219 days," lamented Sen. Mark Kirk, an Illinois Republican whose work with Democratic Sen. Bob Menendez of New Jersey on oil sanctions helped cripple Iran's economy and drive it to the negotiating table.

Kirk pledged to come forward with a new bipartisan sanctions package after the Republican takeover of the Senate. Menendez suggested similar action, saying he'd work "to ensure that Iran comprehends that we will not ever permit it to become a threshold nuclear state."

The U.S. administration strongly opposes additional sanctions because it fears it will push Tehran away from the table.

Monday's decision already appeared to benefit Iran. Its nuclear program is left frozen but intact, without any of the cuts sought by the U.S. And while the negotiations continue, so will monthly dole-outs of $700 million in frozen funds that began under the temporary nuclear deal agreed on late last year that led to the present talks.

Kerry called for patience, saying he hoped congressional skeptics would "come to see the wisdom" of giving talks an extra "few months to be able to proceed without sending messages that might be misinterpreted."

In Tehran, hard-liners fearful that their country will give away more than it gets under any final deal may increase pressure on Supreme Leader Ayatollah Ali Khamenei to break off talks. Still, the latest extension appears to have the approval of Khamenei, the ultimate arbiter in his country.

"Many gaps have been eliminated," Rouhani said in a statement, though he added the sides were "still some distance" from a deal.

U.S.-Iran relations have warmed since Rouhani took office last year.

Still, Rouhani has struggled to sell the idea of negotiating with arch-foe America to hard-liners at home and he pledged "ultimate victory" for the Islamic Republic in securing a favorable agreement.

A joint statement read by Iranian Foreign Minister Mohammad Javad Zarif and EU official Catherine Ashton, speaking for the six powers negotiating with Iran, said the sides "remain convinced that ... a comprehensive solution can be reached."